The C-rating. I won't bother explaining what it means scientifically - that would be boring.
For most people, all you need to know is that 1C is the battery's capacity in mAh divided by 1000.
Therefore, 5C is the battery's capacity divided by 1000, then multiplied by 5. This is the current that you can charge the pack at.
There are plenty of 20A capable chargers out already. I have one. The
iCharger 208B, the
Fusion Emporer 702B, and the
Hyperion 720i
A 50W charger will get you nowhere. The chargers listed above vary from 250W output to 350W output (although the Emporer can charge two batteries with 300W output each at the same time
)
I've been charging my 5S 5500mAh Hyperion pack at 3C - charging in 20 minutes is good!
Of the newest packs where 5C charging is permitted, I'd say 2C or 3C won't affect the pack's lifetime, 4C and 5C may shorten it a touch. But, LiPo cells are good for well over 100 cycles, but few racers will ever give them that before they move to something newer - so lifetime, in my opinion, doesn't matter a great deal.
Basically, when buying a new charger look for these things:
-Charge current, over 10A is good
-Charge power - there's no point in being able to charge at 10A if you can only do so with 1 NiMH cell. Divide the power output by your battery's peak voltage (so 4.2V per cell) to get the charge rate. If the rate is at least twice that of your pack's capacity divided by 1000 then good.
-Voltages supported - up to 6S is really the thing to have - most flexible. Obviously, higher voltage is better, but could be pointless overall. Still look at the power figures.